In recent years, ink jet printers have been utilized widely as a printer for characters and images and also come into use as apparatus for producing electronic devices and deoxyribonucleic acid (DNA) chips. Here, the electronic device refers to an element that utilizes the flow or accumulation of electrons to perform computation, information storage and transmission, display, etc. and a group of these elements. Examples thereof include electric circuits, and wirings, electrodes, resistors, capacitors and semiconductor elements constituting the electric circuits.
In the following, a general outline of the ink jet printer and an example of producing an electronic device using the ink jet printer will be described. The printing mechanism in the ink jet printer is as follows: several picoliters of ink are discharged from each of a large number of penetrating holes (in the following, referred to as “nozzle holes”) with a diameter of several tens of μm provided in a flat plate (in the following, referred to as a “nozzle plate”) toward a printing material such as paper so that the discharged ink is placed at predetermined positions on the printing material. In order to place the ink at predetermined positions on a recording medium, the ink is discharged while moving the positions of the nozzle and the printing material mechanically so as to control the relative positions therebetween. Such a method of discharging liquid (also referred to as liquid drops) from the nozzle holes of the nozzle plate and placing the liquid at predetermined positions on a substrate is called an ink jet method. Also, a device including a mechanism of discharging the liquid from the nozzle holes is called an ink jet head. The ink jet head includes the nozzle plate, the nozzle holes penetrating through the nozzle plate, a pressure chamber that contacts a surface of the nozzle plate opposite to a liquid discharge surface and is in communication with the nozzle holes, and a mechanism to generate pressure in the pressure chamber. By applying pressure to the pressure chamber, the liquid held in the pressure chamber is discharged from the nozzle holes to the outside of the nozzle plate.
FIG. 10 schematically shows the entire ink jet printer. An ink jet printer 100 in FIG. 10 includes an ink jet head 101 utilizing a piezoelectric effect of a piezoelectric element for recording and allows an ink drop discharged from this ink jet head to land on a recording medium 102 such as paper, thus recording on the recording medium. The ink jet head is mounted on a carriage 104 disposed in a main scanning direction X and reciprocates in the main scanning direction X as the carriage 104 reciprocates along a carriage shaft 103. Furthermore, the ink jet printer includes a plurality of rollers (a moving system) 105 for moving the recording medium relatively in a sub scanning direction Y, which is perpendicular to a width direction of the ink jet head 101 (the direction X). The ink jet head is constituted by the nozzle plate provided with the nozzle holes for discharging the ink, a driving part for discharging the ink from the nozzle and a part for supplying the ink to the nozzle.
FIGS. 11A to 11C illustrates an example of a structure of the ink jet head. FIG. 11A is a sectional view showing a nozzle hole 121 and its vicinity. The nozzle hole is in communication with a pressure chamber 113, and a diaphragm 112 and a piezoelectric element 111 are formed above the pressure chamber 113. The pressure chamber 113 is filled with ink, which is supplied from an ink flow channel 115 through an ink supply hole 114. When a voltage is applied to the piezoelectric element 111, the piezoelectric element 111 and the diaphragm 112 flex, increasing the pressure in the pressure chamber 113, so that the ink is discharged from the nozzle 121. The surface of a nozzle plate 116 is treated to be water repellent so that the ink 118 is discharged from the nozzle hole 121 in a constant direction. There also is the case of employing a method of generating air bubbles in an ink chamber for the purpose of increasing the pressure in the pressure chamber 113 (the Bubble Jet (registered trademark) method). FIG. 11B is a schematic three-dimensional perspective drawing taken along a line I-I in FIG. 11A. Although only the structure near about two nozzle holes is shown here, a large number of the same structures are aligned in practice. This figure shows the state in which a left-side piezoelectric element 117 and the diaphragm 112 flex, so that the ink 118 is discharged from the nozzle hole 121 in a direction indicated by an arrow 119. Incidentally, as becomes clear from the figure, one pressure chamber 113 and one piezoelectric element 117 are provided for each nozzle hole, whereas the ink flow channel 115 for supplying the ink is shared among a large number of the nozzle holes, and the ink is supplied from the flow channel through the ink supply hole 114 provided in each of the pressure chambers 113. FIG. 11C is a plan view seen from above the nozzle plate. In this example, there are two rows of 40 nozzle holes 121 aligned from left to right at about 340 μm intervals. In this figure, a line 120 surrounding each nozzle indicates the shape of the piezoelectric element present on the other side of the nozzle plate, and a broken line 124 indicates the shape of the ink follow channel. Since the ink is supplied from one ink flow channel to the 40 nozzle holes aligned from left to right, the ink of the same color is discharged from these 40 nozzle holes. Numeral 122 denotes an arrow indicating a feed direction of a substrate, and numeral 123 denotes the state in which the nozzles are aligned in two rows.
The following is a description of representative examples utilizing the ink jet printer as an electronic device producing apparatus. There is an example in which, by drawing metal colloid on a printed board by the ink jet method, a circuit pattern of lead wires is formed on the printed board (see Non-patent document 1 below). In order to form a lead wire circuit pattern on the printed board, a method usually adopted is a method including forming a metal film on the board and then forming the lead wire circuit pattern by photolithography or a method including forming a negative circuit pattern on the board with a resist film and then forming the lead wire circuit pattern by plating in a region without the resist, followed by removing the resist. The use of the ink jet method is advantageous in that the circuit can be formed on the printed board directly without conducting a troublesome photolithography process. Thus, it takes less time to form the circuit, making it possible to reduce the manufacturing cost considerably. Further, the photolithography requires a photo-mask (plate) corresponding to the circuit to be produced. Therefore, in the case of producing small batches of a variety of circuits and prototyping various circuits, it is necessary to produce a large number of photo-masks, thus consuming more time and cost. On the other hand, since the ink jet method requires no photo-mask, it is suitable for producing small batches of a variety of circuits and prototyping circuits.
Moreover, there are examples in which, by drawing functional organic molecules on a substrate by the ink jet method, a field effect transistor (see Non-patent document 2 below), a display utilizing electroluminescence (see Non-patent document 3 below) and a microlens array (see Non-patent document 4 below) are formed. The functional organic molecular thin film formed on the substrate tends to peel off from the substrate or have deteriorated electric characteristics when exposed to a developing agent or a peeling agent for the resist. Thus, it is difficult to form the pattern by a usual photolithographic process. Since the ink jet method allows the easy formation of patterns without deteriorating the characteristics of the functional organic molecules, it holds out a great promise as the method for producing an electronic device using the organic molecules.
Also, in recent years, as a means of examining a human physical predisposition, a disease diagnosis, an efficacy of a drug or the like at a genetic level, a DNA chip has been in wide use. The DNA chip is obtained by fixing several thousands to several tens of thousands of kinds of DNA fragments or synthetic oligonucleotides (in the following, referred to as “DNA probes”) to a predetermined position on a several centimeters square glass substrate or silicon substrate and is used for determining many gene expressions at the same time or for checking whether or not a specific gene is present. A method of producing this DNA chip by the ink jet method has been suggested. That is, liquid in which the DNA probe is dissolved is placed at a predetermined position on the substrate by the ink jet method, making it possible to form a DNA chip in a simple manner at low cost (see Patent document 1 below).
In order to produce an electronic device or a DNA chip by the ink jet method, a liquid drop has to be placed accurately at a predetermined position on the substrate. In general, initial positions of the ink jet head and the substrate are set, and the liquid drop is discharged while shifting the relative positions between the head and the substrate by a preset amount, thereby placing the liquid at a predetermined position on the substrate. When the pattern of the liquid drops to be drawn is on the order of about several hundreds of μm, this method can achieve an accurate drawing. However, the initial positions and the moving amount of the ink jet head and the substrate vary in the range of μm due to the way the substrate is fixed and thermal expansion of the substrate caused by a temperature change. Therefore, it is difficult to draw a pattern in μm to several tens of μm by the above-described method.
Further, in the liquid discharge using the ink jet head, although with low frequency, the nozzle hole becomes clogged and the liquid is not discharged in some cases. For producing an electronic device and a DNA chip with excellent reproducibility, it also is important to detect whether or not the liquid drop is discharged properly.
Patent document 2 below suggests a spotting apparatus capable of fixing a reactant to a specific position in a detecting portion. In this patent, the position of a substrate is recognized by a vision camera disposed obliquely above the substrate on which the liquid drop is to be placed, thereby placing the liquid on the substrate accurately.
Further, Patent document 3 below suggests a DNA probe solution delivery device including an ink jet head having a plurality of nozzles discharging a DNA probe solution and a means of generating a drive signal for discharging liquid from a specified nozzle provided in the head, characterized by having a light-projecting means for projecting light toward the solution discharged from the nozzle and a light-receiving means for receiving the light from the light-transmitting means. The direction of the emitted light is parallel with a discharge surface of the ink jet head, and the light reflected by the liquid discharged from the head is received, thereby examining whether or not the solution is discharged normally.
Moreover, Patent document 4 below suggests a method for manufacturing an organic electroluminescence display device including, when forming an organic layer by discharging a liquid-phase organic material to a picture element on a substrate by an ink jet method, (a) forming an image recognition pattern on the substrate in advance, (b) recognizing the image recognition pattern using an image recognition device so as to obtain positional information of the substrate or the picture element, and (c) based on the positional information of the substrate or the picture element, controlling a positioning of an ink jet head and the substrate or the picture element and a timing of discharging the liquid-phase organic material. In this document, the image recognition device is disposed and fixed to the other side of the substrate with respect to the ink jet head, and a method of recognizing the image recognition pattern through a transparent or semi-transparent substrate is illustrated. In this conventional example, there is no disclosure about a placement position of the image recognition device with respect to the substrate and the illumination light required for recognizing the image.
From the result of examination conducted so far by the inventors of the present invention, it is understood that the space between the discharge port and the substrate has to be equal to or smaller than 1 mm in order to place a fine liquid drop pattern on the order of several hundreds of μm or smaller on the substrate. This is because, if the space is larger than this, the convection of the air changes the traveling direction of the liquid discharged from the discharge port by the time the liquid adheres to the substrate. Furthermore, if the space is larger, a minute liquid drop sometimes evaporates before adhering to the substrate.
In the apparatus shown in Patent document 2, since the vision camera is disposed obliquely above the substrate, when the space between the discharge port and the substrate is equal to or smaller than 1 mm, the position of the substrate immediately below the discharge port is difficult to see. Especially in the case of an ink jet head having a nozzle plate provided with the discharge ports in high density, it is impossible to detect the position of the substrate immediately below the nozzle holes near the center of the nozzle plate with the vision camera because of the obstruction by the edge of the nozzle plate.
Similarly, in the apparatus illustrated in Patent document 3, since the light parallel with the head has to be irradiated, the smaller space between the head and the substrate makes it difficult to bring the light between them.
In Patent document 4, since the vision camera is disposed on the other side of the substrate with respect to the ink jet head, the region in the substrate in which the liquid drop is to be placed can be observed even when the space between the ink jet head and the substrate is reduced. Now, in order to recognize the positions of the ink jet head and the substrate with the vision camera, it is necessary to project light to the ink jet head and the substrate using a light source and make the light reflected by the ink jet head and the substrate enter the vision camera. However, Patent document 4 fails to disclose how the light source is disposed. In general, a method of putting the light source between the vision camera and the substrate is used. However, this method makes it necessary to increase the distance between the substrate and the vision camera for disposing the light source. In the case where the region in the substrate to be recognized is on the order of about μm, a large-scale optical system is needed for capturing such a minute region with the vision camera, so that the entire apparatus becomes large. Accordingly, the position of the vision camera has to be fixed. The vision camera also is fixed in Patent document 3. In the case where only one of the substrate and the ink jet head moves, the relative positional relationship between the substrate and the ink jet head can be determined easily using a computation processing circuit based on the information obtained from the vision camera. On the other hand, in order to mass-produce electronic devices by the ink jet method, the speed of liquid drop placement has to be raised. Accordingly, it becomes essential to move the ink jet head and the substrate at the same time, while discharging the liquid drops from many nozzle holes at the same time. In this case, since the relative positional relationship between the vision camera and the substrate and that between the vision camera and the many nozzle holes vary from moment to moment, the computation processing circuit for determining the relative positional relationship between the substrate and the ink jet head becomes large. As a result, in the liquid drop placing apparatus of Patent document 4, the optical system and the computation processing circuit become large, leading to an increase in the price of the device.
Patent document 1: U.S. Pat. No. 5,658,802
Patent document 2: JP 2003-98172 A
Patent document 3: JP 2002-253200 A
Patent document 4: JP 2001-284047 A
Non-patent document 1: G. G. Rozenberg, Applied Physics Letters, vol. 81, 2002, p.p. 5249-5251
Non-patent document 2: H. Sirringhaus et al., Science, 2000, vol. 290, p.p. 2123-2126
Non-patent document 3: J. Bharathan et al., Applied Physics Letters, vol. 72, 1998, p.p. 2660-2662
Non-patent document 4: T. R. Hebner et al., Applied Physics Letters, vol. 72, 1998, p.p. 519-521